U.S. Pat. No. 5,106,270 to Goettel et al discloses a two stage compressor and intercooler/aftercooler arrangement for providing pressurized air for the operation of brakes and other devices on locomotives and trains of railway cars connected to such locomotives. As disclosed in the patent, the compressor has two low pressure cylinders and a high pressure cylinder that develop air pressure. Between the high pressure cylinder and cylinder head and the low pressure cylinders and their heads are located intercoolers that cool the pressurized air generated in the low pressure cylinders before such air is sent on to the high pressure cylinder for high pressure air development. A single intercooler core design is also available that collectively receives the air discharged from the low pressure cylinder heads and cools the air before entering high pressure head's inlet flange for the second stage of compression. Air, of course, increases in temperature as it is pressurized. Thus, the need for intercoolers and an aftercooler in the Goettel et al disclosure.
Air produced by the two stage air compressor is usually stored in two main reservoirs located on the locomotive. Maximum pressure provided by the compressor is controlled by a pressure sensitive switch of a governor that is pre-set to regulate the operation of the compressor for loaded and unloaded conditions of the compressor. The compressor normally unloads whenever the main reservoir pressure increases to a "cut-out" pressure setting of the governor pressure sensitive switch. A reduction in main reservoir pressure caused by air use or air leakage, as sensed by a "cut-in" pressure switch setting of the governor, exhausts compressor unloader lines to allow the compressor to again compress air and assume a loaded condition. The governor's pressure sensing switch energizes a magnet valve whenever main reservoir pressure reaches the governor's cut-out pressure switch setting. The magnet valve has an electromagnetic coil that operates the valve. Main reservoir pressure enters the unloader lines of the compressor to unload the compressor and associated intercoolers.
The compressor governor switch is normally located between the number 1 and number 2 main reservoirs in a locomotive.
Each of the cylinders of the compressor in the above U.S. Goettel et al patent is provided with two unloader valves, one for each of two inlet valves of each cylinder, for unloading pressure from cylinder heads when main reservoir pressure increases to the "cut-out" pressure setting of the governor. Main reservoir air is directed to the unloader valves by the magnet valve when its coil receives a voltage signal from the governor's pressure switch. This occurs when electrical contacts of the switch close to energize the compressor magnet valve. Main reservoir pressure, operating through and supplied by the energized magnet valve, moves the unloader valves to unseat an intercooler pressure seal valve (located within unloader valve bodies) and compressor suction valves that are pneumatically connected to the intercoolers. The unseated unloader suction valves prevent the compressor from building air pressure from the ambient outside air taken in by the compressor.
Air is slowly exhausted from the intercoolers whenever the compressor is unloaded by connections between the intercoolers and the high pressure cylinder, i.e., air passes through the unloader valves, past the open intercooler pressure seal valve and through an exhaust port of the unloader valves to atmosphere. Such an exhaust path and procedure requires about twenty-five seconds for the intercoolers to unload their air pressure through the high compression cylinder of the compressor and its unloader valve.
Locomotive compressors are usually driven by an electrical motor having a rotor mechanically connected to the crankshaft of the compressor, though a compressor can be driven directly by the diesel powered engine of the locomotive. When driven by an electrical motor, electrical contactors supply power to the motor whenever compressed air is needed, as ordered by the compressor governor switch. Presently used compressor drive motors are usually a dual speed type. The speed of such motors operate in a predetermined relationship to the speed of the diesel engine of the locomotive, i.e., when the diesel engine speed is between a low idle and some intermediate throttle speed, the compressor motor operates at a high speed configuration, which provides a motor rpm generally twice that of diesel engine speed. When diesel engine speed is between the intermediate speed and a top speed, electrical power is supplied to a low speed configuration of the motor, and the compressor runs at substantially the speed (rpm) of the diesel engine.
The transition time from one configuration of a motor to the other configuration is quite short, on the order of two seconds, such that intercooler pressure may be at its maximum when the transition occurs even though the compressor itself at this time is unloaded. If the intercooler is not unloaded, its pressure is supplied to the high pressure cylinder of the compressor. When a motor transition occurs, the motor starts at the new configuration against any residual pressure in the compressor, such that the motor can be unduly loaded. The motor, in turn, requires an increase in current flow which overheats the motor and shortens its life. The increase in current flow also burns the electrical contactors supplying power to the motor.